This project focuses on biophysical principles of arrhythmogenic mechanisms underlying two inherited diseases: arrhythmogenic right ventricular cardiomyopathy (ARVC) and catecholaminergic polymorphic ventricular tachycardia (CPVT). In both cases, arrhythmias and sudden cardiac death (SCO) develop. However, the specific mechanisms underlying ventricular tachycardia/fibrillation (VTA/F) and SCO in either ARVC or CPVT patients has not yet been resolved. In ARVC, arrhythmias may result from impaired mechanical coupling between cardiomyocytes due to mutations in desmosomal proteins, which may lead to dysfunction of the intercalated disk, and eventual disruption of gap junction plaques, myocyte death and fibro-fatty replacement. In CPVT arrhythmias are the result of abnormal calcium regulation due to leaky mutated ryanodine type-2 receptor channels in the sarcoplasmic reticulum. Yet, it is unknown whether the arrhythmias originate in the 3-dimensional myocardium or in the more isolated, cable-like Purkinje network. Our general hypothesis is that regardless of the mechanism(s) by which arrhythmias are triggered in ARVC and CPVT, the final common pathway in the mechanism underlying VTA/F is wavebreak and reentry. The project combines expertise in cell culture, optical mapping, histopathology, immunohistochemistry and computer modeling to provide testable predictions about how alterations of either structural or Ca2+ regulatory proteins translate into electrical abnormalities that ultimately result in VTA/F and SCO. We propose four Specific Aims: 1) To determine electrophysiological consequences of fibroblast replacement of myocytes and of alterations in intercellular coupling in ventricular constructs and their role in the genesis of reentry in ARVC. 2) To establish the individual roles of alterations in intercellular coupling and fibro-fatty deposits in the genesis of arrhythmias in 3D models of the dysplasic right ventricle. 3) To investigate mechanisms of triggering and maintenance of reentry in biological and numerical models using 2D patterns of CPVT-like mutated mouse cells, mimicking the Purkinje network and the Purkinje-muscle junction. 4) To investigate mechanisms of VT initiation and the transition to VF in simulations using a realistic 3D model of the CPVT-like mutated mouse heart. The proposed work should provide new insight into arrhythmia mechanisms in diseases leading to alterations in the structural and functional homeostasis of the heart.